The present invention relates to an image heating apparatus for heating the image on recording medium. In particular, it relates to an image heating apparatus effective as a fixing apparatus mounted in a copying machine, printer, etc.
Heretofore, as a fixing apparatus employed by an image forming apparatus employing an electrophotographic method, an electrostatic recording method, or the like, a so-called thermal fixing apparatus has been widely used, which comprises a fixing roller and a pressure roller, and which passes the recording medium bearing an unfixed toner image, through the nipping portion formed by the fixing roller and pressing roller, in order to fix the unfixed toner image on the recording medium to the recording medium to effect a permanent toner image.
FIG. 10 is a schematic view of a typical thermal fixing apparatus which employs a heat roller. The fixing roller 40 comprises: a hollow metallic core 42 formed of aluminum, stainless steel, or the like; a heating member 41, such as a halogen lamp, disposed within the hollow of the metallic core 42; and a release layer 43 formed of fluorinated resin, or the like, on the peripheral surface of the metallic core 42 to prevent toner offset. The surface temperature of the fixing roller is detected using a temperature detecting means such as a thermistor, and the power supply to the heating member 41 is controlled by an unshown power supply control circuit, so that the surface temperature of the fixing roller detected by the temperature detecting means 44 remain constant.
A pressure roller 50 comprises a metallic core 51, an elastic layer 52 formed on the peripheral surface of the metallic core 51, and a release layer 53 formed on the peripheral surface of the elastic layer 52. The elastic layer 52 is formed of silicon rubber, sponge formed by foaming silicon rubber, or the like. The release layer 53 is formed of fluorinated resin, or the like, as is the release layer of the fixing roller 40.
The fixing roller 40 and pressure roller 50 are kept pressed upon each other, with the application of a predetermined amount of pressure, so that a nipping portion is formed as a fixing nip N. The two rollers 40 and 50 are rotationally driven so that their peripheral surfaces are moved in the direction indicated by the arrow marks in the drawing. In operation, a recording medium P bearing an unfixed toner image is introduced into the fixing nip N, and is passed through the fixing nip N, remaining nipped by the two rollers 40 and 50. While the recording is passed through the fixing nip N, the unfixed toner image on the recording medium P is permanently fixed to the recording medium P by heat and pressure.
In some of high speed image forming apparatuses, and some of image forming apparatuses which use color toners, an approximately 2 mm thick elastic layer formed of silicon rubber or the like is provided between the hollow metallic core 42 and release layer 43 of the fixing roller 40 of the fixing roller 40, in order to satisfactorily fix the toner, more specifically, in order to prevent the unfixed toner image from being nonuniformly fixed. The provision of this elastic layer softens the peripheral surface of the fixing roller 40. Therefore, as the recording medium P is nipped by the two rollers 40 and 50, the softened peripheral surface of the fixing roller 40 presses the recording medium and the toner particles in the toner image, in a manner to embrace the toner particles on the recording medium P, improving the efficiency with which heat is transmitted from the fixing roller 40 to the recording medium and toner particles thereon.
In recent years, there has been a strong desire for reducing electrical power consumption because of the environmental concern. On the other hand, the market has been demanding higher image quality and higher image output. Thus, in order to meet both the desire for the reduction in electrical power consumption and the market demand for higher speed and higher image quality, various proposals have been made to improve the above described thermal fixing apparatus employing a heat roller.
Referring to FIG. 11, one of the such proposals is disclosed, being realized in the form of a thermal heating apparatus low in power consumption and superior in thermal efficiency, in Japanese Laid-open Patent Applications, 10-301417, 11-073050, etc., according to which in order to reduce the electrical power consumption by reducing the time it takes for the fixing roller 40 to reach the operational temperature, heating rollers 47 and 55 containing the heating members 46 and 54 are placed in contact with the peripheral surfaces of the fixing roller 40 and pressure roller 50, respectively, to heat the peripheral surfaces thereof.
The heating members for heating the peripheral surface of the fixing roller 40 or pressure roller 50 can be divided into two types: a type which is kept in contact with the peripheral surface of the fixing roller 40 or pressure roller 50; and a type which is not placed in contact therewith. The contact type is higher in heat transmission efficiency. Further, in terms of another heating roller property different from the preceding ones, the heating rollers can also be divided into two types: a type which contains the heating members 46 and 54, as shown in FIG. 11, and a type, in which the internal surface of its hollow metallic core is coated with a heat generating resistive layer, that is, an electrically resistive layer which generates heat as electrical current is flowed through it, with the interposition of an electrically insulative layer formed of organic resin (polyimide, for example), glass, or the like material.
As an example of the latter type, a thermal fixing method of a film heating type is proposed in Japanese Laid-Open Patent Applications 63-313182, 2-157878, 4-44075, 4-204980, etc. According to this thermal fixing method, a toner image on recording medium is fixed through a piece of film which is thin and low in thermal capacity, and which is placed between the heating portion and pressure roller, and power consumption is minimized by not supplying power to the thermal fixing apparatus while an image forming apparatus is on standby. FIG. 12 shows the general structure of an example of a film heating type fixing apparatus. In the drawing, a referential code 60 stands for a film assembly, which comprises: a heater 61 comprising a plate of ceramic, such as alumina, aluminum nitrate, etc., and a heat generating resistive layer formed on the ceramic plate; a stay/holder 62 which is formed of heat resistant resin, and to which the heater 62 is solidly fixed; a heat resistant thin film 63 (which hereinafter will be referred to as fixing film) which is formed of polyimide, or the like, and which is loosely fitted around the stay/holder 62. The heater 61 of this film assembly 60, and the pressure roller 50, are kept pressed against each other, with the interposition of the fixing film 63, forming the fixing nip N.
As the pressure roller 50 is rotationally driven in the direction indicated by an arrow mark, the fixing film 63 is conveyed by the rotation of the pressure roller 50 in the direction indicated by another arrow mark, through the fixing nip N, while being kept in contact with the heating roller 61 (sliding on the heating roller 61). The temperature of the heater 61 is detected by a temperature detecting means 64, such as a thermistor, disposed on the back surface of the heater, and is fed back to an unshown power supply control portion so that the temperature of the heater 61 is kept at a predetermined level (fixing temperature).
Compared to image forming apparatuses, such as printers, copying machines, etc., employing a thermal heating apparatus in accordance with the prior art, which employs a thermal roller, the image forming apparatuses employing a thermal fixing apparatus of a film heating type such as the above described one enjoy various advantages in that they are higher in heating efficiency and shorter in startup time, making it unnecessary for the heating apparatus to be kept warm (preheated) during a standby period, and also, shortening the wait time.
However, the above described thermal fixing apparatuses of various types all have their merits and demerits. For example, in the comparison between a thermal fixing apparatus of a heat roller type and a thermal fixing apparatus of a film heating type, the former is superior to the latter in process speed, durability, etc.
In order for a thermal fixing apparatus to be preferably employed by a high speed image forming apparatus, even a thermal fixing apparatus employing a heat roller must have an elastic layer. However, a conventional thermal fixing apparatus employing a heat roller is larger in the thermal capacity of its fixing roller. Therefore, the time it takes for the surface temperature of the fixing roller to be raised to a predetermined level by the heat transferred from within the fixing roller through the elastic layer, which is inferior in thermal conductance, is overwhelmingly long, compared to the thermal heating apparatuses employing a film heating type fixing method. Further, a conventional thermal fixing apparatus employing a heat roller is greater in the amount of the electrical power it requires to raise the temperature of the fixing roller to a predetermined level, and is greater in the wait time, that is, the length of the time from when the electrical power source of an image forming apparatus is turned on to when the image forming apparatus becomes ready for an actual printing operation. Also, it requires that the temperature of its fixing roller is kept warm even during a standby period. Therefore, they consume a greater amount of electrical power.
In comparison, in the case of a thermal fixing apparatus in which the peripheral surfaces of the fixing roller and pressure roller are heated from outside, as shown in FIG. 11, by heating members (heat rollers) which are relatively small in thermal capacity, it is unnecessary to heat the interior of the fixing roller having a large thermal capacity; only the peripheral surface portion of the fixing roller must be heated. Therefore, it takes less time and a smaller amount of electrical power for the surface temperature of the fixing roller to be readied to the predetermined level. However, even this type of thermal fixing apparatus is far inferior to a thermal fixing apparatus employing a film heating type fixing method, because in the case of the former, which is structured so that its heating means, such as a halogen heater, is disposed within its hollow metallic core as shown in the drawing, the interior of the heat roller itself must be heated before the peripheral surface of the fixing roller begins to be heat, and therefore, it take a longer time for the peripheral surface of the fixing roller to be raised to the predetermined level.
Further, a thermal fixing apparatus employing a heat roller structured so that a heat generating resistive layer is disposed on the electrically insulative layer coated on the internal surface of the heat roller, starts up faster than the aforementioned thermal fixing apparatus, the temperature of which is raised by a halogen heater. However, in the case of a thermal fixing apparatus employing this type of heat roller (heating member), the area in which the peripheral surface of the fixing roller can be supplied with heat is only the heating nip formed as the fixing roller and pressure roller are kept pressed upon each other. In other words, in the areas other than the heating nip in which the heat roller is virtually in contact with the peripheral surface of the fixing roller, a substantial amount of the heat from the heat roller radiates into the ambient air, reducing the efficiency with which the heat is supplied to the peripheral surface of the fixing roller. In addition, the fixing roller of this type is also larger in thermal capacity, compared to a fixing roller of a film heating type. Therefore, the fixing roller of this type must also be supplied with a certain amount of heat during a standby period.
In an image forming apparatus in which recording medium is conveyed at a slow speed, the amount of the heat to be supplied to the fixing roller does not need to be as large as the amount of the heat to be supplied to the fixing roller in an image forming apparatus in which recording medium is conveyed at a higher speed. Therefore, in the case of the former image forming apparatus, even a heater roller is satisfactory as a heating means. However, in the case of the latter image forming apparatus, that is, the apparatus in which recording medium is conveyed at a higher speed, it is impossible for the heat roller to supply the fixing roller with a sufficient amount of heat, because of heat loss, that is, because a substantial amount of the heat from the heat roller radiates into the ambient air from the area of the peripheral surface of the heat roller other than that in the heating nip.
Because of the above described reasons, such a thermal fixing apparatus that does not consume electrical power during a standby period, is substantially shorter in the length of time from the reception of a print signal to when the fixing apparatus becomes ready for thermally fixing an unfixed toner image on recording medium to the recording medium (which hereinafter may be referred to as first print time) than a thermal fixing apparatus in accordance with the prior art, and is capable of allowing an image forming apparatus to operate at a higher speed while satisfactorily fixing images inclusive of halftone images, has not been realized, and the development of such a thermal fixing apparatus has long been desired.
The present invention was made in consideration of the above described problems, and its primary object is to provide an image heating apparatus which is suitable for an image forming apparatus high in the output per unit of time in terms of the copy count, and yet, is low in electrical power consumption.
Another object of the present invention is to provide an image heating apparatus which is suitable for an image forming apparatus higher in the output per unit of time in terms of copy count, and yet, is shorter in the first print time.
Another object of the present invention is to provide an image heating apparatus comprising: a rotational member; backing means for forming, in combination with said rotational member, a conveying nip through which recording medium is conveyed; and heating means for heating the peripheral surface of said rotational member, said heating means having a heater, in the form of a plate, for forming, in combination with said rotational member, a heating nip.
These and other objects, features, and advantages of the present invention will become more apparent upon consideration of the following description of the preferred embodiments of the present invention, taken in conjunction with the accompanying drawings.